[intro]

Reid: We've tried and failed to go to Mars...so many times. Don't get me wrong; we *have* completed successful missions to Mars. I mean, it's right next door. So we've racked up both victories and let's call them... learning opportunities. But the sheer number of attempts may or may not excuse the fact that half of Mars missions fair. There must be something about Mars that makes it really hard to get to, and that's the Martian mystery we're about to dive into, along with the cool innovations bringing up our success rate. So, to get you settled in, here's why it's so hard to land on Mars.

[slide: "Why It's So Hard To Land on Mars"]

Thanks to its watery history and potential for past life, Mars has been fascinating people for decades. So it's no surprise that we've sent more spacecraft there than any other planet - we're talking 45 missions! Most other worlds have had just a small handful. The problem is around half of the probes that have ever attempted to explore Mars have either crashed or disappeared. So as much as we want to understand the planet, getting to its surface is no easy feat. Mars' unique atmosphere often gets the better of us, and it's taken some creative engineering to get to the ground.

Before Mars, the only places we'd ever landed were the Moon and Earth, and while that did come with challenges, we had strategies nailed down pretty well for both. The hard thing about landing on Earth is that our thick atmosphere creates extreme friction and heat with incoming spacecraft, but we've solved that problem with heat shields. And besides, that thick atmosphere also means parachutes work very well.

The Moon is kind of the opposite; it has virtually no atmosphere, which gets rid of the heat problem, but it also means parachutes don't work. We have to use retrorockets to land - little rockets that fire underneath the spacecraft to slow its descent.

Mars, meanwhile, is a whole different beast. It comes with all the challenges of landing -

- on Earth *and* the Moon, but with none of the real benefits. Its atmosphere is 100x thinner than Earth's, meaning parachutes can't grab onto enough air to completely slow down the spacecraft. But unlike the Moon, there's also just enough atmosphere to create problems. Just like friction causes space rocks and old satellites to burn up in Earth's atmosphere, a space probe entering Mars' atmosphere can get hotter than 2000°C; that's hot enough to melt iron and just about every other metal, so the millions of dollars worth of machinery we send to Mars need serious protection to keep from being fried. So, how do you get an expensive, heavy chunk of metal traveling tens of thousands of kilometers an hour to come gently to a stop on the surface of another world? A whole lot of creativity. And probably a good amount of coffee.

Evey mission to land on Mars starts with something called an "aeroshell," a special capsule that protects its cargo against the heat. Its outer layer is filled with a material called an "ablator" that was invented in the 1970s for the first Mars landers the Viking Missions. It reacts with the Martian atmosphere in a way that removes the heat and leaves behind a trail of gas. It gets so hot that it glows red, but inside the capsule, cargo stays a little cooler than room temperature.

Next, once friction has slowed things to about 1,600 km/hr, a parachute opens and part of the aeroshell is cast off. Amazingly, engineers are still using a parachute pretty similar to the one designed for the Viking landers more than 40 years ago; it's made of nylon and polyester with tethers made of the same material as bulletproof vests. That makes it super strong and light, which is really important considering the craft is still moving at supersonic speeds when it deploys. And while it isn't enough to slow down a spacecraft all the way, it does help. After a few minutes, the parachute brings the craft down to a few -

- hundred km/hr, and it gets discarded along with the rest of the aeroshell. 

Now, this is where things get really creative, and no type of mission has been exactly the same. Engineers have had to come up with special solutions to get each spacecraft on the ground. For example, those 1970s Viking landers used retrorockets like on the Moon. But there was always the possibility that they'd botch a landing on uneven ground, and while they were fine for landers, carrying around a bunch of rockets would be a pointless burden on the rovers we started sending to Mars in the 90s.

So for the Pathfinder Mission that landed in 1997, which included the first experimental rover, engineers tried a new method - a cluster of airbags. After slamming into the ground, this robotic explorer bounced along for hundreds of meters. And by "bounced," I mean it shot several stories into the air and moved as fast as cars on the freeway before rolling to a stop. But somehow it worked. In fact, it worked so well that scientists used the same system to land the more recent Spirit and Opportunity rovers.

Then in 2012, things had to change again because airbags were out of the question for the Curiosity rover; it was nearly 5x the mass of Spirit and Opportunity, so engineers came up with their most epic solution yet: They called it a "sky crane." Basically, it was a stage that used retrorockets to hover above the surface. From there, it slowly lowered the rover on a tether, then cut itself free, and flew off to crash land nearby. NASA's next Mars mission, Mars 2020, will use a similar strategy. But who knows what kind of unique designs we'll see after that.

Oh, and in case this all isn't complicated enough, every single step of these landings also has to happen completely automatically. That's because radio signals travel at the speed of light, so they take *at least* eight minutes to go from Earth to Mars and back, -

- which is longer than it takes to land. And it's not exactly easy to put a spacecraft on autopilot in a world that's still really foreign and unpredictable. 

The good news is all these years of work have been well worth it. Besides preparing us for future exploration, these landers have brought us closer to knowing what Mars was like in the past. That could help us figure out whether or not it ever hosted life, and what it would take to support *human* life one day in the future. And it's all thanks to some brilliant engineers and organizations.

[slide]

As it turns out, there's a lot of factors making Mars' surface tough to touch down on. But ultimately, we have surpassed these obstacles and landed there. Here's Caitlin to tell us what it took to do it the very first time.

[slide: "The First Time We Landed on Mars"]

Caitlin: Before every Mars landing, there's a tense period of radio silence between the moment the spacecraft hurdles into the atmosphere and the moment it touches down. During what's now known as the "seven minutes of terror," the spacecraft slows from several km/s to a complete stop all on its own.

These days, scientists have the help of sophisticated computers, AI, and fancy gadgets like self-piloting sky cranes. But back when scientists made the first successful landing on Mars, those minutes of silence were *especially* terrifying. That first landing didn't earn a big place in history books, and yet the strategy behind it paved the way for many truly historic missions to Mars.

This was all back in May of 1971 when Soviets launch the Mars 3 mission, which included a lander and an orbiter. It launched just days after its twin, Mars 2, and set off on the six-month journey to Mars. The plan was for the orbiters to take photos of the Martian surface from on high and return information about things like topography and magnetic fields. They'd also relay communications from the landers to Earth. Meanwhile, the landers would take pictures from the surface and measure properties like wind speed, air pressure, and temperature. They even contained an itty bitty rover on skis that would study the soil.

That might all sound like a tall order -

- considering that at this point the Soviet Union hadn't even done a flyby of Mars, but they'd done some similar science on the moon, so these types of experiments weren't entirely new.

The part that *was* new was the landing. And landing on Mars...is not easy. The first problem is that its atmosphere is *super* thin. Spacecraft are traveling several km a second when they arrive, so they need to slow down a lot. But a parachute isn't that effective when it doesn't have a lot of air to grab onto. Unfortunately, at atmosphere is also still thick enough that the spacecraft will burn up if they're not carefully protected. And the whole descent has to happen automatically, because the spacecraft gets all the way to the ground faster than a single signal can travel from the earth. So, it's all on its own, and if the slightest thing goes wrong, a Mars landing can be doomed.

That's what happened with Mars 2; it entered the atmosphere at too steep of an angle and plunged straight into the surface. But it did earn the honor of being the first spacecraft to reach the surface of Mars. Technically. And a few days later, its twin became the first to stick the landing.

Mars 3 got off to a better start; after it split ways with its orbiter, it entered the atmosphere at a super shallow angle, less than 10 degrees. that made it possible for Mars 3 to take advantage of a technique called "aerobraking." Basically, it used the drag from the atmosphere to help it slow down. Then it deployed two parachutes, starting with a small pilot chute, then a larger main chute. the main chute would tear if it opened too soon, so at first, a cord around the base kept it from fully inflating in a technique called "reefing." Then, once it got below supersonic speeds, it popped open.

At this point, the altimeter turned on: that's a tool that uses light to calculate distance by bouncing radar signals off the ground and measuring their return time. And using this, the descent module could kick off its final landing sequence right before it hit the ground. By then, it was still going more than 60 meters per second, so, a split second away from disaster, it tossed off its parachute and fired its retrorockets. Even so, it slammed into the ground at more than 20 meters per second, -

- more than 20 meters per second, or about 45 miles per hour. But it was designed to; shock absorbers inside protected the cargo, and when everything came to a stop, four petal-like hatches opened to reveal the lander. Unfortunately, it only transmitted for 20 seconds and sent back a grand total of one grainy image. As far as we can tell, it had the misfortune of landing in a severe dust storm, and that may have done it in.

So Mars 3 never really went down in history, but in a lot of ways, it was an important success. For one, the orbiter sent back pictures of the surface, revealing huge martian mountains and providing the data for topographical maps. And it collected data about the atmosphere, gravity, and magnetic fields of the red planet. But the lander was important, too. Not only did it mark a milestone and prove that we *could* land on Mars, it taught us how. It showed us how a bunch of sophisticated, automatic maneuvers could be combined to pull off a pretty soft landing. And while it's not simple, this sequence of steps replace the need for lots of heavy fuel and other landing hear that might overburden the craft.

These days, the basic steps for landing on Mars are mostly the same. Recent missions also used aerobraking parachutes, retro rockets, and radar to make their way to the surface. The main difference is that as payloads have gotten heavier, engineers have designed bundles of airbags and elaborate sky cranes to assist with the final touchdown.

By now, humans have pulled off Mars landings 10 different times in the last half century. And while Mars 3 didn't give us glamorous photos or discoveries to remember it by, its success is embedded in every single mission that comes after it.

[slide]

Reid: After seven minutes of terror, we managed to land on Mars. And only a special kind of person would hear the phrase "seven minutes of terror" and think, "Hmm...let's do that again."

So it's a big deal to perform another liftoff after landing, but when the alternative is trying to get over the extreme terrain to explore the red planet, sometimes another launch is the best option. In this case, it's a much smaller craft... Okay, it's a drone, -

-  but technically, this was the first helicopter on mars. And back in 2017, Hank was in the throes of the successful lead up to that launch.

[slide: "NASA Might Send a Helicopter to Mars"]

Hank: Over the last 50 years, we've sent tons of cool spacecraft to Mars: flybys, orbiters, landers, rovers, seems like we've done everything. Still, there's one kind of mission that we have not done yet - no Mars mission has flown through the air. That might change in just a few years.

Nothing's final yet, but engineers are experimenting with the idea of including a drone called "the Mars helicopter", on the upcoming Mars 2020 rover. Mars 2020 is NASA's successor to Curiosity, and it's expected to launch in - you guessed it - 2020. It has a similar design to Curiosity and will study potentially habitable environments. It will also select and package samples that we could return to Earth on a future mission.

Adding a helicopter could help this rover overcome one of Curiosity's biggest problems - it just doesn't go that far. Curiosity's been on Mars for over five years, but in that time, it's only driven about 17.5 km or an average of less than 10 meters per day. Part of that is because the rover stops and studies things, but it's also because driving a rover on another planet is pretty dang hard. Radio communication with Mars takes anywhere from 8 to 48 minutes round-trip, depending on where the Earth and Mars are in their orbits, so Mission Controllers can't drive Curiosity Mario-Kart style. It can steer itself across simple terrain like a self-driving car, but it still needs to stop every now and again to get input from Earth, and there are some kinds of rocky or difficult terrain that it just can't handle.

Picking Curiosity's path isn't always the easiest either. To decide where it should go, engineers rely on pictures from the rover and from satellites in orbit, but Curiosity's cameras can only see so far, and the satellites have a top-down view, so they can't always see the true shape of surface features. If Mars 2020 could launch a drone to scout out the area ahead, it could anticipate obstacles and identity the most interesting things to study -

- and someday, a Mars helicopter - or "Marscopter" - might even be able to explore places a Rover couldn't reach, like small channels or cliffs.

This all sounds like an amazing idea, but there's a big problem: Mars is not a very good place to fly. Helicopters stay in the air because they experience lift, or more pressure underneath them than above them. And the more dense the air is, the more lift your helicopter can get, because there's more air molecules for it to push against.

The problem is Mars' atmosphere is really thin - like, less than 1/60 the density of Earth's atmosphere at sea level, so it's a lot harder to create lift. But - good news - there's less gravity, so that's one thing working in favor or us, but it is not enough to counteract just the lack of molecules to push against.

For this to work, engineers would have to give their Marscopter extra long rotor blades. Basically, this would let the helicopter push against more air molecules at once, even if they're spread further apart. To carry just one kilogram across the martian surface, the mars helicopter would need rotors more than a meter across, which is a lot bigger than your neighbor's photography drone. And that doesn't mean the drone could *carry* one kilogram of samples either; everything from the rotors, to the flight computer, to the solar panels, would have to add up to a kilogram of mass.

But amazingly, getting airborne might actually be one of the easiest parts of a Mars helicopter. Remember that communications delay between Earth and Mars? Well, unlike a rover which can sit around and wait for instructions, once the Mars helicopter is airborne, the clock's tickin'. It would probably fly for two to three minutes and could cover up to half a km of terrain, but since we wouldn't be able to steer it in real-time, every second of that would have to be on autopilot. It would have to take off, judge the wind speed, fly in the right direction, take pictures, and find a safe place to land, all in 180 seconds or less.

That might seem like a lot of work, but it could come with a big payoff. NASA engineers estimate having a Mars helicopter could help a rover like Mars 2020 travel 3x farther than Curiosity in a day, and when you're talking about a multi-billion dollar mission, -

- tripling efficiency is a pretty sweet deal. As a bonus, all those extra near-surface images would be really helpful for scientist studying Mars. You'd get a mission that could study not only *more* targets, but better ones, and that's a heck of a good thing for exploration. 

So far, NASA has already tested a prototype of the helicopter design, but they'll need to do a lot more work before they're ready to start zooming around. Since Mars 2020 is expected to launch in less than three years, hopefully, we'll be hearing more about it soon.

[slide]

Reid: On April 19th, 2021, that helicopter did us proud. And since then, it's covered more than seven miles of Martian terrain from the sky.

So, we've innovated tech to get around the inhospitable Martian rocks, but we're not going to let the robots have all the fun. Here's how *we* could go to Mars, too.

[slide: "the Electric Thruster That Could Send Humans to Mars"]

When you imagine humans on their way to Mars, you probably imagine them with a spacecraft with big, explosive chemical engines, and that's totally reasonable. Humans need to travel with a lot of stuff, and engines that rely on chemical combustion are currently the only one powerful enough to move us through space at a reasonable speed. Except chemical engines also have a pretty big downside; they need to carry a bunch of fuel, which makes their spacecraft super heavy. And that leads to more expensive missions that are harder to launch. Honestly, it would be nicer if we could move humans with a lighter, more fuel-efficient propulsion system, and the good news - we might have already found our best option.

It's a form of electric propulsion called a "Hall-effect thruster" or a "Hall thruster" for short. They're thrusters that look a bit like a bullseye and glow with an eerie, colorful light. And they could be the future of human space exploration. Unlike some of the ideas we talk about on this channel, Hall thrusters aren't theoretical or even new; they were invented in the 1960s, and engineers have spent decades advancing the technology.

On a basic level, these thrusters work by accelerating charged particles called "ions." First, they start with a circular channel -

- or a few channels depending on how big your thruster is. Between each channel, you put some magnetic coils that generate a magnetic field. And then, at the bottom of your channels, you add an electrically charged plate called an "anode," which creates an electric field. And finally, you add a cathode, which is located somewhere outside the channel and can spit out a bunch of electrons. And now, you're ready to go.

When you power up the thruster, the cathode starts releasing those electrons, and the particles are attracted to the anode, so they go flying into the channel. There, they're caught up in the magnetic field and start zooming in circles around and around the thruster, and that's where the magic happens:

Once the electrons are zooming around, Hall thrusters pump a bit of propellant into the channel, usually a neutral gas like xenon. The xenon gets git with all those incoming electrons, and that knocks off some of its electrons and turns the xenon atoms into ions. The electric field inside the thruster then pushes those xenon ions out of the channel at incredible speeds, sometimes more than a dozen kilometers per second. And that's what generates the thrust to move your thruster and your spacecraft forward.

Now, this basic idea of accelerating ions isn't unique to Hall thrusters; every form of ion-based electric propulsion does something like this. What makes Hall thrusters special is that they satisfy three major conditions:

For one, they have among the highest thrust of all forms of electric propulsion. There are a few reasons for this, but one is the propellant ions are created and accelerated in the same area. Other thrusters keep these processes separate, and there's a limit on how many charged particles they can cram into one spot before the electric field gets messed up.
Hall thrusters also use their fuel really efficiently. Since they accelerate their ions to such high speeds, they generate more thrust for every molecule of propellant they use.

And finally, they can fire for a long time. Other ion thrusters have components that quickly wear out, -

- and while Hall thrusters *do* have their own lifetime problems, engineers have found ways to solve or mitigate many of them. So in the end, these thrusters can fire for much longer, which means they're a lot more practical for space flight.

Since the 60s, Hall thrusters have flown on dozens of missions, mostly they've been used to adjust satellites' orbits, but in the 2020s, they'll be used on even bigger projects, like the Psyche mission to investigate an asteroid.

But earlier, I said Hall thrusters could be the future of human space exploration, and the thrusters we have today... well, they're nowhere near strong enough to push around people at a helpful speed. Because there's the thing about Hall thrusters and about electric thrusters in general: Their main benefit is that they can fire for a long time. That means though you might start off slow, you can gradually build up speed until you're zooming along faster than any spacecraft that uses chemical propulsion. The problem right now is that getting to those speeds takes a long time. With our current tech, it would take years to get people to Mars.

But someday, that could change, because there's a Hall thruster currently in development that could become strong enough to move humans. It's called "X3." It's been in development since 2009, and it gets its name because it has three channels instead of the more-common two. This allows it to accelerate more ions at once. It's still nowhere near strong enough to fly humans even if we put several of them on the back of a spacecraft, but it has generated more thrust in the test than any other Hall thrusters.

Now, engineers are working to make X3 more reliable and increase its thrust, and if they can manage that, NASA may eventually select the thruster to help send people to Mars. Even if this doesn't happen though, there's a good chance this project will help inspire other teams to continue the work. There are a lot of electric propulsion methods out there, and many of them are already changing space flight. But when it comes to flying humans around, Hall thrusters might be our best bet.

At the end of the day, a lot of people just want to see humans walk on the surface of Mars. But while that will be amazing, it's worth remembering the -

- engineering behind this goal, too. It's taking a lot of clever, creative work to make something like this feasible, and the research done on Hall thrusters is a great example of that. 

[slide]

So, getting people to Mars is another challenge. We need thrusters that are energy efficient, go fast, and get to that speed quickly, otherwise the trip is going to take a long time... like, months one-way. But here's an idea from years ago that could shorten it to 40 days.

[slide: "The VASIMR Engine: How to Get to Mars in 40 Days"]

Someday, we're going to send people to Mars, and it's going to be awesome. But for now, everyone from NASA to Elon Musk is still trying to figure out the best way to do it. With today's rockets, a one-way trip to Mars takes somewhere around seven months, but one company is developing an engine that might be able to get us there in only 40 days. It's called the VASIMR engine, and it propels spacecraft using a jet of plasma.

VASIMR stands for "VAriable Specific Impulse Magnetoplasma Rocket," and the idea behind it has actually been around since the 1970s. But engineers didn't make too much progress on it until 2015 when NASA gave the Ad Astra company a grant to help develop it as part of their NextSTEP program. It works using a kind of electric propulsion, and it's a big step up from the other engines available right now.

To get your rocket into space, you'd still need a chemical engine, which generates thrust with the reaction that releases tons of gas, like combining hydrogen and oxygen. So far, that's the only kind of engine we have that's powerful enough to get a heavy rocket to space. But once you'er there, you can move it with all kinds of things, from ions to particles of light, depending on what your mission is.

Right now, any spacecraft that would send people to Mars would still use a chemical engine because we mostly have the technology figured out. But carrying all that fuel also carries a lot of weight, so newer kinds of engines use electric propulsion, often in the form of ion thrusters, which create a beam of charged atoms -

- or ions - to push your spacecraft around. Ion engines are a lot more efficient than chemical engines, but they aren't designed to handle large payloads, like a bunch of humans and all the supplies they'd need for a trip to Mars. So, we mostly use ion thrusters for small satellites.

VASIMR also uses electric propulsion, but it will be *way* more powerful. It propels the spacecraft by creating plasma in the form of a super hot jet of ions and electrons. Unlike other kinds of electric propulsion, it uses radio waves to heat the plasma rather than electrodes or other electronics in ion engines, which - among other things - makes the engine a lot more durable.

To make the plasma, it starts by pumping a gas like hydrogen or argon into a tube which is surrounded by a magnet and two couplers, a kind of device that emits radio waves. Most kinds of gas will work, which makes VASIMR really versatile, but hydrogen is a good choice if you want a lightweight, easy-to-find fuel. Once the gas is pumped in, the first coupler strips some electrons off their atoms and turns the gas into plasma. At this point, it's already very got - around 5,500 degrees Celsius - but then the second coupler makes it even hotter. It raises the plasma to 10 million °C, which isn't that far from the temperature of inside the Sun.

A magnetic nozzle then turns that super hot plasma into a nice, controllable jet, and it's show out of the end of the engine. Besides being more durable, VASIMR is great because it also has different settings, like the gears on a car, which means it can generate the right amount of thrust for different kinds of missions. So besides getting people to Mars, it could also be scaled down to send small satellites zooming around the Earth.

But most importantly for our future astronauts, VASIMR can be made big and powerful enough to move human-sized spacecraft. Ion engines just aren't ready to do that yet, partly because many tend to rely on more mechanical parts than VASIMR, some of which aren't designed -

- to work on a large scale, although engineers are working on that, too. 

Still, there is one thing we need to figure out before VASIMR is ready to go - the power supply. It takes a lot of power to produce all those radio waves, especially if you want an engine strong enough to push a crew of astronauts. Solar power can generate enough power to propel small satellites, but we'd need something a lot stronger for a full-size spacecraft. And our best option is probably using a small nuclear reactor, but because those have the potential to be really dangerous, we'll want to make sure that we're extra confident in that technology before we start using nuclear power to transport people through space.

Right now, the goal for the VASIMR team is to develop an engine so it can fire for 100 consecutive hours at 100 kilowatts of power, which is 10 to 100x more power than an ion thruster has. To get to Mars in 40 days, you'd still need a lot more power than that - like, 2000 times more power - and that's probably where a nuclear reactor would come in. But for now, 100 kilowatts is a good start. Ad Astra's plans with NASA are to have the engine ready for the 100 kilowatt goal by the summer of 2018. As of last August, they'd fired the engine for around 10 hours, so they're making progress. Still, even after they meet that goal, VASIMR will need to go through plenty of development and tests before we use it to go anywhere.

So when the first human steps on Mars, we'll probably have made it there with a chemical engine since that technology is a lot more developed, but someday, our trips to Mars could be a whole lot faster.

[slide]

Since that 2018 video, the VASIMR engine has completed its power-endurance test but hasn't made it to Mars yet. So while we're waiting, could we shorten that travel time? Here's another innovation that could - theoretically - get us to Mars in just *three* days.

[slide: "Photonic Propulsion: Mars in 3 Days?"]

There's been all kinds of buzz lately about some new space tech that could send a spacecraft to Mars in three days, and maybe even get spaceships to -

- exoplanets that are light-years away, which kind of sounds like something out of a science fiction novel. But a group of researchers from the University of California, Santa Barbara is working on a new way to travel in space.

Known as "photonic propulsion," it would use a giant set of lasers to push ships along, and if it works, it could eventually be used to explore other star systems. But that's a big *if*. The project is called "DEEP-IN," and its goal is to use electromagnetic acceleration to get ships close-ish to the speed of light, fast enough that interstellar travel could actually make sense. These days, our spaceships use chemical acceleration; in other words, they burn fuel, and they *are* fast. They're just not fast enough to travel to other stars in any reasonable amount of time, which is where electromagnetic acceleration comes in.

Instead of using chemical energy to push itself forward, a DEEP-IN spacecraft would use the energy from electromagnetic radiation, more specifically, the energy from a huge set of lasers powered by sunlight. That laser array technology is a whole project all by itself called the "Directed Energy System for Targeting of Asteroids and exploRation," or DE-STAR, and being developed by the same team of researchers. And as you might have noticed from the first part of that acronym, DE-STAR would be a laser system with multiple uses, like, destructive uses.

DE-STAR is mainly being designed to protect Earth, both by diverting asteroids that are headed for us and by vaporizing space debris. This photo-driven propulsion thing is just a bonus, and a pretty huge one. It's based on the idea that light has a lot of pushing power, which comes from its momentum. We might not feel that push from just walking around on Earth, but a giant reflector screen in space *does* feel it. That's the science behind solar sails, and we've already built spaceships that use them.

When photons in light bump into a solar sail, their momentum is transferred and the spacecraft is propelled forward a little bit. DE-STARS' lasers would provide lots of light that we could use to take the solar-sails concept one step further and start building spacecraft that use laser sails. See, solar sails are limited because light coming from the sun only comes with so many photons.

But this laser sail will have concentrated beams of photons shooting directly at it - it's the difference between a sprinkler and a fire hydrant. 

When it gets hit by that laser beam, the laser sail (and anything that happens to be attached to it, like a spaceship) is going to start zooming through space. A ship using a laser sail wouldn't have to carry as much fuel, which would mean that it could have a much lower mass. It could also, in theory, go very, very fast. With a huge laser array putting out 50-70 gigawatts of power, a 100 kilogram ship (about the size of Voyager 1) could travel at around 1.5% the speed of light, nearly 300 times Voyager's top speed.

But not surprisingly, there are still challenges to solve when it comes to making spaceships powered by giant lasers. We *could* send a smaller probe to Mars in three days or a larger craft on a trip that would take about a month. But we'd need a giant, square laser array that's 10 kilometers long on each side, which presents some obvious problems. Getting stuff to space is expensive, let alone 100 square kilometers worth of high-powered laser equipment. And even if we get everything to the right spot, it would be incredibly difficult to assemble.

Building a laser sail for the super-fast trip to Mars would also be tough. According to researchers, it would have to be only a micron thick, that's a thousandth of a millimeter. But to work properly, that whisper-thin laser sail would have to weight about as much as the spaceship itself, meaning that it would have to be a huge, thin sail, but strong enough to be stable while the ship is moving ridiculously fast. And we also have no idea how to slow the spacecraft down once it gets to wherever it's going. So if we're talking about sending ships to Mars in three days, we're probably getting ahead of ourselves a little bit here.

Really, the first step to making this research a reality is building a much smaller set of lasers. The team's plan, assuming NASA chooses to move forward with their idea, is to start with a laser array that's only one meter square, and then we'd just keep building bigger sets of lasers until we eventually figured out how to build one that's 10 km on a side. Then, we could use that to launch what are known as "wafer sats." These miniature spacecraft would weigh -

- no more than a gram, but they'd have sensors, a power source, teeny-tiny thrusters, and communications equipment.

That giant set of lasers could accelerate those wafer sats to about 20% of the speed of light, sending them light years away, where they could tell us about interstellar space and exoplanets. They might even be able to reach the nearby star system, Alpha Centauri, only 15 years after they launch.

So, there's still a huge amount of research, technology development, and testing needed before we can use lasers to propel any sort of spacecraft, even a little wafer one, and it's going to be a very long time before we're zooming over to Mars in just a few days. But it probably is possible.

[slide]

From the time it takes to *get* to Mars, to the difficult landing maneuvers that its terrain requires, Mars poses a lot of new challenges. Maybe it's no surprise that so many missions fail. But leave it to the rocket scientist to make the impossible possible.

This video was made possible thanks to our friends over at Patreon. Our patrons support the work we do on this channel, and in exchange, they get access to a ton of perks: like access to our exclusive discord, bloopers from our video shoots, and our behind-the-scenes podcast. If that sounds like a good deal, you can head to patreon.com/scishow to sign up or just to learn more. And thanks for watching.

[outro]